1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display which can adjust the luminance of a backlight according to external illuminance.
2. Discussion of the Related Art
In general, a liquid crystal display (hereinafter, LCD) has a trend that its application scope has been gradually widened due to its lightness, its thinness, and its low power consumption. In accordance with such a trend, the LCD is used in an office automation device, an audio/video device and the like. The LCD adjusts the transmittance quantity of a light beam in accordance with an image signal applied to a plurality of control switches arranged in a matrix to thereby display desired pictures on a screen.
Since the LCD device is not a self-emission type display device, the LCD device requires a light source such as a backlight. A fluorescent lamp, such as a cold cathode fluorescent lamp (CCFL) or external electrode fluorescent lamp (EEFL), or a light emitting diode, is used as the light source.
Recently, there are proposed backlight control methods which can expand the luminance range of a displayed image by adjusting the brightness of a backlight in accordance with a change in external illuminance. In these backlight control methods, a photosensor is mounted on a liquid crystal display panel to sense the illuminance of external light. Based on this sensing information, if the external illuminance is high, the brightness of the backlight is increased, and if the external illuminance is low, the brightness of the backlight is decreased, thereby achieving a reduction in power consumption in a low illumination environment and preventing a decrease in visibility in a high external illumination environment.
The photosensor is a TFT (thin film transistor) device that turns on in response to an external light, and determines the level of an output voltage, which is an illuminance sensing information, by increasing the amount of electric charges discharged through itself according to the amount of received light. A gate voltage (for example, a voltage lower than a threshold voltage for an N-type TFT and a voltage higher than a threshold voltage for a P-type TFT) for not turning on the photosensor by an external driving voltage is supplied at a constant level to the gate electrode of the photosensor. This gate voltage serves as a bias voltage.
However, when a gate voltage of the same polarity is applied for a long time to the gate electrode of the photosensor, the operating characteristics of the photosensor are varied. This is because a threshold voltage level of the photosensor is shifted by a gate-bias stress. In FIG. 1, a rise (Vth0→Vth1) in threshold voltage due to a shift of the operating characteristics to the right is caused by a positive bias stress, while a fall (Vth0→Vth2) in threshold voltage due to a shift of the operating characteristics to the left is caused by a negative bias stress.
Such a change in operating characteristics with time (hereinafter, “time-varying characteristics”) of the photosensor causes an increase or decrease of electric current discharged through the photosensor under the same illumination condition. FIG. 2 shows one example in which electric current discharged through the photosensor under the same illumination condition increases due to time-varying characteristics. In FIG. 2, the longitudinal axis indicates electric current Iph discharged through the photosensor, the horizontal axis indicates a drain-source Vds, the dotted line indicates an initial state, and the solid line indicates a state after time variation.
Resultantly, a difference in discharge current amount due to such a change in time-varying characteristics leads to a deviation in output sensing voltage to thus degrade the accuracy of illuminance sensing, and, moreover, becomes a major factor in varying the brightness of a backlight with time regardless of the same illumination condition.